The present invention relates to an improved pneumatic actuator for oil well pumps which is operated by the pneumatic pressure of the wellhead gas or by an external air source. In more detail, the present invention relates to a pneumatic actuator that is secured to a wellhead for reciprocating either hollow or solid sucker rods and used in conjunction with a wide variety of reciprocating plunger-type oil well pumps.
The present invention is intended for use in the same application as the pneumatic pump actuator described in my U.S. Pat. No. 3,643,432 and is an improvement on that device. That device is used on wells having standard strings of rods and tubing or wells having a single tubing string, and its construction is shown in FIG. 1 of that patent. That device requires the securing of a mounting frame assembly (reference numeral 11 in the figures of that patent) including a bottom plate 14, side support structure 15 and cylinder base plate 16, and the vertical dimension of side support structure 15 must exceed the length of the stroke of the piston 19 in cylinder 17 to allow travel of hollow tubing 23 and polished rod 18 up and down to discharge fluid through outlet 51. Both mounting frame assembly 11 and cylinder 17 are installed on top of the well casing 12, making the entire unit very tall (the '432 patent describes an actuator built in accordance with that patent having a 36 inch stroke; when the height of mounting frame assembly 11 and the other parts of the device are added to that 36 inch stroke, total height may exceed 7 feet) and therefore, hard to work on.
The pump actuator described in that patent and also, for instance, in my U.S. Pat. Nos. 3,782,247 and 3,986,355, uses a solid polished rod 18 which extends upwardly through the mounting frame assembly 11 and on up to the piston 19 where it is secured to the piston 19 by a nut 21 on the end of the rod 18. That structure requires that piston rod 18 project through an aperture in the cylinder base plate 16 such that piston rod 18 must be sealed against air or gas leakage by a packing gland 22. Although such pump actuators having given years of satisfactory service, packing gland 22 has proven to be troublesome because it is hard to keep it from leaking. Further attempts to reduce leakage by replacing packing gland 22 on a regular basis are hampered by the difficulty, resulting from the structure of the device, of replacing the packing gland 22.
Further, although not necessarily described in the specification of that patent, the device described in the '432 patent was designed to be used on wells having a wide range of depths and pressures, and was, therefore, designed with high pressure applications in mind. Of course, if designed for that purpose, the device must include certain structure, e.g., for internal braking, reversing controls, and so on, all made with high pressure components. The result is a unit which is relatively expensive to build, install, and maintain. Such expense is a problem in the case of many wells which are marginal producers such as, for instance, the shallow stripper wells so common throughout the southern portion of the great state of Texas. Such wells, many of which produce less than, for instance, two barrels of oil per day, are such slow producers that the replacement cost of new rods, tubing and pumping units is prohibitive. Consequently, stripper wells in Texas, and throughout the world, are being plugged and abandoned when that equipment wears out in spite of the fact that they may be steady producers and may have relatively large reservoirs. There is, therefore, a need for a pump actuator which is inexpensive to build, install and maintain, and which will operate reliably and efficiently, and it is a primary purpose of the present invention to provide an improved pump actuator which meets those criteria.
Another object of the present invention is to provide a pump actuator which is adapted for use in connection with other production equipment designed with this same principal objective. For instance, production of oil through hollow sucker rods eliminates the need for at least one complete set of downhole tubing, thereby decreasing costs, and the present invention is intended for use with such rod strings. The present invention is also intended to take maximum advantage of the cooperation possible between pump and pump actuator in that the pump actuator is intended for use in connection with the hollow rod, downhole pump described in my co-pending application Ser. No. 07/518,166. For instance, that pump avoids sanding and gas lock problems by contacting the standing valve with the traveling valve during the downstroke as the plunger nears the maximum extent of downward travel in the barrel. For proper operation of such a pump, stroke length is important, and the present invention provides a pump actuator in which stroke length can be adjusted in seconds, even during production operations. As an example of how such combinations can make old stripper wells profitable, such a system can produce oil efficiently from depths of up to about 1500 feet with a 11/2 inch bottom hole pump through 1 5/16 inch tubing using an eight inch I.D. power cylinder and 36 inch stroke length with air compressed to less than 50 psi. Many stripper wells are not much more than 300 feet apart, and such wells therefore lend themselves to being operated from a single source of compressed air using pump actuators constructed in accordance with the present invention which have an installed cost of about $300 each, a dramatic cost reduction compared to conventional equipment. These costs and operating efficiencies can make an entire once-marginal field profitable again.
Other advantages of the present invention will be apparent to those skilled in the art from the following detailed description of a presently preferred embodiment of a pneumatic pump actuator constructed in accordance therewith.